Sensor element for determining the concentration of a gas component in a gas mixture

ABSTRACT

A sensor element for determining the concentration of a gas component in a gas mixture, in particular for determining the concentration of oxygen in the exhaust gas of an internal combustion engine, having a laminated body that is made up of a plurality of solid electrolyte layers and that has an upper and a lower layer that are each fashioned as a film, as well as an intermediate layer. In order to simplify the manufacture and to reduce the manufacturing costs, the films of the upper and lower layer are fashioned with equal thicknesses, and the intermediate layer is applied as at least one printed layer of a film binder.

FIELD OF THE INVENTION

The present invention is based on a sensor element for determining theconcentration of a gas component in a gas mixture, in particular fordetermining the concentration of oxygen in the exhaust gas of aninternal combustion engine.

BACKGROUND INFORMATION

In a known sensor element of this type (German Patent Application No. DE199 41 051), an upper, lower, and intermediate layer are cast as ceramicfilms of a solid electrolyte material that conducts oxygen ions, such asyttrium- stabilized zirconium oxide. A further solid electrolyte layer,situated between the upper and intermediate layers, is applied forexample onto the upper layer by screen printing of a pastelike ceramicmaterial. The material used for this is preferably the same as that ofwhich the upper layer is made, thus e.g. zirconium oxide. In the plane(surface) of the additional solid electrolyte layer, there are situatedfunctional layers, such as electrodes of a Nernst cell and a pump cell,and a reference duct. Between the lower and intermediate layers there isalso situated an electrical resistance heater, embedded in an insulatinglayer of aluminum oxide. The integrated form of the planar ceramiclaminated body is manufactured by laminating together the ceramic films(which are printed with the additional solid electrolyte layer and thefunctional layers) of the upper, lower, and intermediate layers, usingfilm binders and subsequent sintering.

SUMMARY OF THE INVENTION

The sensor element according to the present invention has the advantagethat the inventive construction of the laminated body results in theomission of one film, and therewith of two film binder layers. Thelamination layers of the laminated body are reduced from two laminationlayers in the known sensor element, namely between the upper layer andthe intermediate layer on the one hand and between the lower layer andthe intermediate layer on the other hand, to only one lamination layerremaining between the lower and upper layer, thus reducing theprobability of gas leaks in the lamination area and lowering the rejectrate. Because the number of cast films significantly influences theproduction costs of the sensor element, these costs are also reduced byomitting a film. In addition, two additional printing steps are omittedfor the application of the film binders for the intermediate layer,further reducing the manufacturing costs.

For the upper and lower layer, films having uniform thickness are used,so that in the manufacturing process only a single casting charge needbe produced, so that the films have the same dry contraction(shrinkage), green contraction, and sintering contraction. The use ofthicker films avoids handling problems, such as those that occur whenscreen printing is used to print the functional layers on thinner films.The film of the upper layer, which is thicker than that in the knownsensor element, additionally provides the advantage that in the film,which is made mostly of zirconium oxide, the pumping of oxygen throughthe zirconium oxide has the result that fewer slight breakdowns, in theform of electrically conducting paths that could falsify the measurementsignal, occur over the long-term, so that the serviceable life of thesensor element increases.

According to an advantageous specific embodiment of the presentinvention, the upper layer film contains a gas entry hole thatcompletely penetrates through this film, and which is made in the filmbefore the binding of the upper layer film to the laminated body. Theprovision of the gas entry hole in the upper layer film, which is notyet connected to the lower layer film, simplifies the manufacture of thegas entry hole, because the gas entry hole can be made by penetratingthrough the upper layer film, rather than having to maintain a definedboring depth in the laminated body, as is the case in the known sensorelement.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE shows a longitudinal section of a sensor element fordetermining the concentration of oxygen in the exhaust gas of aninternal combustion engine, in a schematic representation.

DETAILED DESCRIPTION

The sensor element for determining the concentration of oxygen in theexhaust gas of an internal combustion engine, shown schematically inlongitudinal section in the Figure as an exemplary embodiment of ageneral sensor element for determining the concentration of a gascomponent in a gas mixture, has a laminated body 10 made up of a lowerlayer 11, an intermediate layer 12, and an upper layer 13. Here, lowerlayer 11 and upper layer 13 are cast ceramic films of a solidelectrolyte material, preferably of yttrium-stabilized zirconium oxide(ZrO₂), while intermediate layer 12 is made of at least one film binderlayer made of a solid electrolyte material that is printed on one of thefilms, for example on the film of upper layer 13, using screen printing.Zirconium oxide paste is preferably used for the film binder layer. Aplurality of film binder layers are applied in successive printingsteps.

The two films for upper and lower layer 13, 11 are constructed with thesame thickness, and have a thickness between 0.3 mm and 1.0 mm, whilethe intermediate layer, made of one or more film binder layers, is madesignificantly thinner, its thickness being selected between 25 μm and100 μm. In a preferred exemplary embodiment, the thickness of the twofilms for upper and lower layer 13, 11 is 0.5 mm in each case, and thethickness of intermediate layer 12 is 50 μm. In the film of upper layer13 a gas entry hole 14 is made, e.g. stamped or bored, that completelypenetrates the entire thickness of the film.

In a known manner, there is fashioned in laminated body 10 a pump cellmade of a solid electrolyte and an inner and outer pump electrode, aswell as a Nernst cell made of a solid electrolyte and a Nernst andreference electrode. The solid electrolyte of the pump cell is formed byupper layer 13, and the solid electrolyte of the Nernst cell is formedby intermediate layer 12. Outer pump electrode 15 and inner pumpelectrode 16 of the pump cell are each printed coaxially to the gasentry hole 15 on the upper or lower side of upper layer 13. Nernstelectrode 17 of the Nernst cell is situated, together with pumpelectrode 16, in a hollow space 23 that is embedded in intermediatelayer 12, while reference electrode 19 of the Nernst cell is situated ina reference gas duct 18 in intermediate layer 12. Reference gas duct 18,which can be charged with a reference gas in a known manner, can befilled with a porous material. Reference electrode 19 is printed,together with inner pump electrode 16, on the lower side of upper layer13. Hollow space 23 is produced by a pore formation layer applied onintermediate layer 12 using a screen printing method. Inner pumpelectrode 16 and Nernst electrode 17, which is likewise situated coaxialto gas entry hole 14 and is axially at a distance from inner pumpelectrode 16, stand in connection in hollow space 23 with gas entry hole14 via an annular diffusion barrier 20, so that outer pump electrode 15can be charged directly with the exhaust gas of the internal combustionengine, and inner pump electrode 16, as well as Nernst electrode 17, canbe charged with this gas via diffusion barrier 20. Between lower layer11 and intermediate layer 12 there is situated an electrical resistancetheater 21 that is embedded in an electrical insulating layer 22, madefor example of aluminum oxide (Al₂O₃). Insulating layer 22 is printed onthe upper side of lower layer 11. Laminated body 10 is manufactured bylaminating together the ceramic films, printed with the film binderlayers and the functional layers, and subsequent sintering of thelaminated structure.

1-10. (canceled)
 11. A sensor element for determining a concentration ofa gas component in a gas mixture, comprising: a laminated body includinga plurality of solid electrolyte layers, the plurality of solidelectrolyte layers including an upper layer, a lower layer and anintermediate layer, each of the upper and lower layers including aceramic film, the upper and lower layers having an equal thickness, theintermediate layer including at least one film binder layer.
 12. Thesensor element according to claim 11, wherein the sensor element is fordetermining a concentration of oxygen in an exhaust gas of an internalcombustion engine.
 13. The sensor element according to claim 11, whereinthe at least one film binder layer is printed on one of the films forthe upper and lower layers.
 14. The sensor element according to claim11, wherein the at least one film binder layer is composed of azirconium oxide paste.
 15. The sensor element according to claim 11,wherein the thickness of the upper and lower layers is between 0.3 mmand 1.0 mm in each case, and a thickness of the intermediate layer isbetween 25 μm and 100 μm.
 16. The sensor element according to claim 11,wherein the thickness of the upper and lower layers is 0.5 mm in eachcase, and a thickness of the intermediate layer is 50 μm.
 17. The sensorelement according to claim 11, wherein the upper layer includes a gasentry hole that completely penetrates the upper layer and that is madebefore a lamination of the laminated body.
 18. The sensor elementaccording to claim 17, further comprising, in the laminated body, a pumpcell having an outer and inner pump electrode situated on a solidelectrolyte, and a Nernst cell having a Nernst electrode and a referenceelectrode situated on a solid electrolyte, and wherein the upper layerforms the solid electrolyte of the pump cell and the intermediate layerforms the solid electrolyte of the Nernst cell.
 19. The sensor elementaccording to claim 18, further comprising a diffusion barrier forconnecting the inner pump electrode and the Nernst electrode with thegas entry hole.
 20. The sensor element according to claim 18, furthercomprising, in the intermediate layer, a reference gas duct that ischarged with a reference gas, and that is in connection with thereference electrode, the reference gas duct being filled with porousmaterial.
 21. The sensor element according to claim 11, furthercomprising an electrical resistance heater embedded in an insulatinglayer and situated between the lower layer and the intermediate layer.